The crocodile icefish or white-blooded fish (Channichthyidae) are a family of perciformfish found in the cold waters around Antarctica and southern South America. Water temperature can drop to -1.9°C (the freezing point of seawater) in the Antarctic sea, but stays rather constant. About 25 species of crocodile icefish are currently recognized. They feed on krill, copepods, and other fish.

Icefish reach a total length of 25–75 cm.

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Their blood is colorless because it contains no hemoglobin.[2][3]Red blood cells are usually absent and if present are rare and defunct.[4]Oxygen is dissolved in the plasma and transported throughout the body without the hemoglobin protein. The fish can live without hemoglobin because of their low metabolic rates and the high solubility of oxygen in water at the low temperatures of their environment.[2] However, the oxygen-carrying capacity of their blood is less than 10% that of their relatives with hemoglobin.

To compensate for the loss of hemoglobin, they have larger blood vessels (including capillaries), greater blood volumes (four times that of other fish), bigger hearts, and greater cardiac outputs (fivefold greater) compared to other fish.[2] Their hearts lack coronary arteries and the ventricle muscles are very spongy, enabling them to absorb oxygen directly from the blood they pump.[5] Their hearts, large blood vessels and low-viscosity (RBC free) blood are specialized to carry out very high flow rates at low pressures.[6] This helps to reduce the problems caused by the lack of hemoglobin. In the past, their scaleless skin had been widely supposed to help absorb oxygen. However, current analysis has shown that the amount of oxygen absorbed by the skin is much less than that absorbed through the gills.[5] The little extra oxygen absorbed by the skin may play a part in supplementing the oxygen supply to the heart[5] which receives venous blood from the skin and body before pumping it to the gills.

Channichthyidae are the only known vertebrates without hemoglobin, an oxygen transport protein in the blood. Although they do not manufacture hemoglobin, remnants of hemoglobin genes can be found in their genome. The hemoglobin protein is made of two subunits (alpha and beta). Almost all of the alpha and beta subunit genes have been lost from the genomes of 15 of the 16 icefish species.[7][8][9] In only one of the icefish species, Neopagetopsis ionah, there is a more complete, but still nonfunctional hemoglobin gene.[10]

Myoglobin, an oxygen transport protein used in muscles, is absent from all icefish skeletal muscles. In 10 species, myoglobin is found in the heart muscle, specifically ventricles.[11] Myoglobin has been lost[clarification needed] in icefish heart ventricles at least four separate times and by four different mechanisms.[2]

The loss of hemoglobin was initially supposed to be an adaptation to the extreme cold; the higher solubility of O2 reduces the demand on hemoglobin and the lack of RBCs decreases the blood viscosity. However, current analysis has shown the lack of hemoglobin—though not lethal—is still maladaptive.[2] The fish have evolved fairly drastic changes to their physiology to compensate. These compensations include spending twice as much energy pumping blood compared to other fish.[2]

These fish have descended from a sluggish demersal ancestor. The cold, well-mixed, oxygen-rich waters of the Antarctic Ocean provided an environment where a fish with a low metabolic rate could survive even without hemoglobin—albeit less efficiently. During the mid-Tertiary period, a species crash in the Southern Ocean opened up wide range of empty niches to colonize. Despite the hemoglobin-less mutants being less fit, the lack of competition allowed even the mutants to leave descendants that colonized empty habitats and evolved compensations for their mutations. Later, the periodic openings of fjords created habitats that were colonized by a few individuals. This gave the opportunity for four lines of fish to even lose their myoglobin genes by a similar process.[2]